P. E. A. Turchi

Systematic study of stacking fault energies of random Al-based alloys

Abstract We have determined intrinsic stacking fault energies, formation energies and structural energies differences for Al–X alloys, where X=Cu, Ag or Mg, from self-consistent electronic structure calculations, using the Layer Korringa Kohn Rostoker method within the coherent potential approximation. We find that the intrinsic stacking fault energy is almost equal to twice the energy difference between the h.c.p. and f.c.c. structures, which indicates that the inter-atomic interactions that are relevant for the structural properties are of short-range. A modified rigid band model is used to understand the origin of the non-linear variation of the stacking fault energy in Al–Cu and Al–Ag with composition. For the Al–Zn system we show that conclusions drawn from the rigid band model hold for any system in which Al is alloyed with an element that has a filled d-band.

Stability of materials

Engineering materials with desirable physical and technological properties requires understanding and predictive capability of materials behavior under varying external conditions, such as temperature and pressure. This immediately brings one face to face with the fundamental difficulty of establishing a connection between materials behavior at a microscopic level, where understanding is to be sought, and macroscopic behavior which needs to be predicted. Bridging the corresponding gap in length scales that separates the ends of this spectrum has been a goal intensely pursued by theoretical physicists, experimentalists, and metallurgists alike. Traditionally, the search for methods to bridge the length scale gap and to gain the needed predictive capability of materials properties has been conducted largely on a trial and error basis, guided by the skill of the metallurgist, large volumes of experimental data, and often ad hoc semi phenomenological models. This situation has persisted almost to this day, and it is only recently that significant changes have begun to take place. These changes have been brought about by a number of developments, some of long standing, others of more recent vintage.

Thermodynamics of the Cr-Ta-W system by combining the Ab Initio and CALPHAD methods

Abstract The thermodynamic properties of alloys can be described from the knowledge of the underlying lattice and the atomic numbers of the alloy species by using a first-principles electronic structure approach based on the tight-binding linear muffin-tin orbital method within the coherent potential approximation (TB-LMTO-CPA) and the local density approximation of density functional theory. The generalized perturbation method (GPM) permits direct mapping of the configurational part of the effective one-electron Hamiltonian onto an Ising-like model thus insuring the necessary link between quantum mechanics and statistical thermodynamics. To test the practical application of this approach to multi-component alloys, data assessment has been successfully performed for the Cr-Ta-W alloy system by using the thermodynamic results derived from the TB-LMTO-CPA-GPM and the cluster variation method (CVM) in the tetrahedron approximation for Ta-W alloys as functions of temperature and concentration. These later results predict B2 ordering for the bcc-based Ta-W system with a maximum ordering temperature near 1000 K at 43 at.% Ta. The output thermodynamics were converted to a Redlich-Kister/Bragg-Williams format with an acceptable level of accuracy. The results were then combined with those of the CALPHAD description of the Cr-W and Cr-Ta systems to calculate isothermal sections of the ternary phase diagram of the Cr-Ta-W system.

A first-principles study of phase stability in NiAl and NiTi alloys

Abstract The thermodynamic stability of the b.c.c.- and f.c.c.-based ordered phases of NiTi and NiAl has been studied with a highly accurate first-principles electronic structure method. The occurrence of a martensitic transformation in NiAl B2 ordered intermetallic alloys will be discussed and related to the existence of intermediate structures between b.c.c.- and f.c.c.-based phases. It will be shown that closely related ordered structures can exist on f.c.c. and b.c.c. lattices in the composition range where the transformation occurs. A variety of antiphase boundaries in the B2 and L12 structures will be examined and the pertinent energies will be compared with experimental data. In addition, the rather unusual appearance of the NiTi B2 ordered structure in the phase diagram will be discussed.

A numerical algorithm for the solution of a phase-field model of polycrystalline materials

We describe an algorithm for the numerical solution of a phase-field model (PFM) of microstructure evolution in polycrystalline materials. The PFM system of equations includes a local order parameter, a quaternion representation of local orientation and a species composition parameter. The algorithm is based on the implicit integration of a semidiscretization of the PFM system using a backward difference formula (BDF) temporal discretization combined with a Newton-Krylov algorithm to solve the nonlinear system at each time step. The BDF algorithm is combined with a coordinate-projection method to maintain quaternion unit length, which is related to an important solution invariant. A key element of the Newton-Krylov algorithm is the selection of a preconditioner to accelerate the convergence of the Generalized Minimum Residual algorithm used to solve the Jacobian linear system in each Newton step. Results are presented for the application of the algorithm to 2D and 3D examples.

Molecular dynamics simulations of a − SiC films

Empirical molecular dynamics simulations combined with a recursion procedure are applied to the study of the atomic and electronic structures of

Theoretical study of phase stability in Ni-Al and Ni-Ti alloys

a\text{\ensuremath{-}}\mathrm{SiC}

Molten Salt Fuel Version of Laser Inertial Fusion Fission Energy (LIFE)

thin films. The films are generated from the condensation of diluted

Atomic and electronic structures of a-SiC:H from tight-binding molecular dynamics

\mathrm{Si}\text{\ensuremath{-}}\mathrm{C}

Vibrational properties of Ga-stabilized δ-Pu by extended x-ray absorption fine structure

vapor on a crystalline silicon substrate similarly to atom-by-atom deposition. The as-deposited films are annealed at different temperatures. Growth kinetics, bonding configuration, chemical ordering, cohesion, relaxation effects, surface roughness, atomic level stress, and electronic properties of the films are investigated as functions of the deposition parameters: vapor temperature, applied particle force, and substrate and annealing temperatures. The results are compared with those associated with bulk and film samples of